Vendored: Yoga flexbox layout engine

This folder is vendored third-party code. Do not hand-edit it.

It is a C# line-port of Meta's Yoga flexbox engine, authored by Microsoft as part of the Reactor project. Both grants are MIT; the full text of both is in THIRD-PARTY-NOTICES.md at the repo root.
Upstream https://github.com/microsoft/microsoft-ui-reactor
Tag v0.1.0-preview.13
Commit c9191b97c40a2e4d6bcbc72df7714184862b4d36
Upstream path src/Reactor/Yoga/
Imported 2026-09-04
Files 10 of the 11 in that folder (FlexPanel.cs, the WinUI adapter, is not vendored — we write our own)

Why it is here rather than referenced

Microsoft.UI.Reactor ships as a single net10.0-windows10.0.22621 package with a WindowsAppSDK dependency and the whole Reactor framework attached, so a PackageReference is not viable for a multi-target Uno library. See specs/flexpanel-import/spec.md (risk 5).

Why it is not under Controls/

It is not a control, and it must not be mistaken for our own code. The Uno.Toolkit.UI XAML merge (xamlmerge-toolkit.props) only globs Controls/** and Behaviors/**, so Layout/ is inert there and no csproj change was needed — the implicit Compile glob picks these files up.

Adaptations applied on import

Every one is mechanical and scripted, by build/scripts/import-yoga.py. Re-running it against a newer upstream reproduces this folder exactly.

  1. Provenance header. Four lines prepended to each file: upstream repo, tag, commit, that file's original upstream path, import date, and SPDX license with both copyright holders. This offsets every file by 4 lines when diffing against upstream — account for it.

  2. Namespace. Every file → Uno.Toolkit.UI.Yoga, and upstream's redundant self-using is dropped.

    ⚠️ Uno.Toolkit.UI.Layout — which specs/flexpanel-import/spec.md originally proposed — cannot be used. It collides with the existing public enum Layout in Uno.Toolkit.UI (Helpers/ResponsiveHelper.cs), and that enum is public API that cannot be renamed.

  3. Explicit using directives added. Upstream builds with ImplicitUsings=enable; Uno.Toolkit.WinUI.csproj does not. System, System.Collections.Generic, System.Runtime.CompilerServices, System.Diagnostics and System.Threading are added per file as needed. The script skips any a file already declares.

  4. FlexLayoutDirection.LTR/.RTL renamed to .LeftToRight/.RightToLeft (spec D3b). The enum is internal now, so this is purely cosmetic; it is kept because it matches WinUI's FlowDirection.LeftToRight and dropping it would churn every corpus file. Numeric values are not renumbered, including FlexDirection's Yoga ordering (Column = 0, ColumnReverse = 1, Row = 2, RowReverse = 3).

  5. FlexEnums.cs made internal. Upstream's six flex enums are the only public types in the folder. Here every top-level type is internal, so an upstream re-sync can never reshape the package's public API. FlexPanel's public enums are our own, in Controls/FlexPanel/FlexEnums.cs, and Controls/FlexPanel/FlexEnumMapping.cs maps them onto these by name. The script's self-check fails on any public top-level type left in the engine.

  6. Nothing else. No reformatting, no #nullable pragmas, no analyzer suppressions — none turned out to be necessary.

Accepted as-is

  • YogaAlgorithm is a static class holding private static uint s_currentGenerationCount, documented upstream as thread-unsafe and bumped via Interlocked.Increment. Layout runs on the UI thread on every Uno target, so the invariant holds; it is a value type, so it pins nothing and is ALC-safe. Recorded rather than refactored — divergence from upstream has to be paid for (spec D8).
  • CSS Grid properties present in C++ Yoga are absent from this port. YogaStyle still carries JustifyItems/JustifySelf, but grid layout is not implemented and is out of scope.

Conformance tests

The upstream generated corpus is ported alongside the engine to src/Uno.Toolkit.RuntimeTests/Tests/Yoga/ — 26 files, 590 test methods (544 active, 46 skipped upstream), 17,784 assertions. It is the reason this engine was imported rather than hand-written, so keep it green. See that folder's README.md.

Re-syncing

  1. Clone microsoft/microsoft-ui-reactor at the new tag.

  2. Bump TAG, COMMIT and IMPORTED in build/scripts/import-yoga.py, then run it:

    REACTOR_SRC=/path/to/reactor python build/scripts/import-yoga.py all

    It rewrites both the engine and the test corpus, is idempotent, and self-verifies — it fails on any leftover upstream namespace, Xunit reference, [Fact], un-renamed LTR/RTL, or missing provenance header.

  3. Build, then run the tier-1 corpus. A conformance failure after a sync is an engine regression, not a test problem.

Re-sync deliberately, never opportunistically — upstream is a 0.1.0-preview and moves weekly.